There is no certainty about the display of an E-reader.  Size, color, and resolution vary.  Plus, a number of E-readers are in black and white.  An “ink & wash” style works well, as does an ink drawing.  Strong red, blue, and green colors will display as black on the black & white E-reader.  Using light color or a toned background frames the outer edges of the picture panel.  Make a to-size reduction in your art program to check details.